Scott manley lands a lunar module on 8-bit silicon: the zx spectrum refuses to die
While the rest of us swipe at glass slabs with more compute than Apollo, Scott Manley just steered a virtual spacecraft onto the Moon using a 3.5 MHz toaster that still smells of 1982 basement plastic. The ZX Spectrum—48 KB of RAM, rubber keys, and a reputation for cassette tantrums—completed a real-time descent burn and touched down without crashing the stack. The trick? Manley never asked the machine to draw pixels; he made it solve orbital trigonometry in its head and stream the coordinates back to a host PC that sketched the scene.
How a 40-year-old cpu became a flight computer
The program, written in Sinclair BASIC, treats the Z80 like a headless sensor box. Altitude, velocity vector, remaining dV—every 200 ms the Spectrum spits a compact string over the 1983-vintage RS232 port on the Interface 1. A Python shim on the modern side catches the packet, projects the state vector into 3-D, and renders the LEM in wireframe. The Speccy never sees a polygon; it only crunches numbers so tight they fit in 16-bit registers. Two seconds of lag separate throttle-up and visual feedback, yet the loop is stable enough to hover, translate, and kill the rate before the contact light blinks.
Manley borrowed the math from 1985's Elite: eight vertices, rotation matrices, perspective divide done with fixed-point shifts to dodge floating-point ROM calls. The lunar terrain is a single affine grid; collision is a height lookup, not a mesh query. Result: 14 lines of BASIC handle guidance, another 20 the telemetry formatter, and the whole binary still leaves 6 KB free for the splash screen.
Why bother when a phone can run ksp natively?
Because the Spectrum is a teaching scalpel. Strip away texture shaders and multithread physics and you see the bare feedback loop that keeps actual spacecraft alive: sense, decide, actuate, repeat. Manley’s stream overlays the numbers on the video feed so viewers watch delta-v dwindle in real time; the same columns once glowed green on CRTs in Houston. The stunt also proves that salvage hardware can join the simulation stack—handy for cubesat clubs that need a dirt-cheap avionics testbed.
Retro hackers have already forked the code. One Madrid group swapped the lunar sphere for Mars, another is porting the uplink to ESP32 so the flight computer can ride inside a 1-U chassis. The bottleneck is no longer CPU; it’s patience—waiting for the 10-minute .tap file to load from a thrift-store Walkman.
Manley closes the video by rebooting the Speccy straight into a game of Jetpac, as if to remind us: the same silicon that once landed pretend astronauts still knows how to have fun. The machine is obsolete only if we stop asking it new questions.